The present invention relates generally to a process and an apparatus for the production of hollow bodies from thermoplastic material, the wall of which comprises a laminate formed by at least first and second layers, by means of extrusion blow molding.
In a process for the production of a hollow body from thermoplastic material with a wall in the form of a laminate structure comprising at least first and second layers, preforms are first produced in a batch-wise manner, the wall of each preform having a suitable number of layers, bu using an extrusion unit comprising at least first and second continuously operating extruders and a common extrusion head. The extrusion unit is provided with a storage chamber for storage of the materials which are plasticised in the extruders, together with means for emptying the storage chamber to form preforms, said means being movable with a stroke-like movement between first and second limit positions. Flows of material from the storage chambers, the number thereof corresponding to the number of layers forming the wall of the hollow body, are brought together in the region of the extrusion head in such a way that mutually adjoining flows of materials are joined together in a laminate-formation area to provide a laminate for constituting the wall of the preform. The laminate is advanced through a communicating duct within the extrusion head to an outlet opening disposed at a spacing from the storage chamber. A given portion by volume of the laminate is then ejected through the outlet opening to form the preform which has two end portions and a central portion, at least a part of the central portion being expanded within the mold cavity of a split blow molding mold under an increased internal pressure therein, while the end portions of the overall portion of material forming the preform are squeezed off the preform, as constituting excess material, by means of a squeezing-off operation in which parts of the blowing mold are involved, the excess material remaining outside the mold cavity of the mold.
In an operating procedure which is similar to that outlined above, as disclosed in German laid-open application (DE-OS) No 36 35 334, the individual flows of material are firstly put into an annular configuration in cross-section, within an annular piston forming part of the extrusion head, and the flows of material are then brought together within the annular piston to form the laminate. The laminate is passed through an annular duct into the storage chamber so that the plastic materials for producing the preform are already stored in laminate form. Thus, in the subsequent emptying stroke movement for emptying the material from the storage chamber, which is performed by the annular piston, the laminate which has already been formed therein is displaced towards the outlet opening.
In another operating procedure as disclosed in German laid-open application (DE-OS) No 36 20 144 (corresponding to U.S. Pat. No. 4,758,144), use is made of an extrusion head in which the flows of material from the extruders are passed through annular ducts disposed in a stationary component of the extrusion head, to a laminate-formation region rom which the laminate produced from the flows of material is passed through an annular duct into the storage chamber which can then be emptied by an annular piston unit forming part of the extrusion head.
The fact that the preform is produced in a batch-wise manner in dependence on the stroke movements of the annular piston for emptying the storage chamber in the above-discussed apparatuses means that a distinction can be made between two operating phases within the extrusion head. After conclusion of the production of a preform by the ejection of a suitable amount of laminate structure through the outlet opening of the extrusion had, the storage chamber is emptied. In that situation, the annular piston takes up its limit position at the end of the emptying operation. In the subsequent phase in operation of the apparatus, the storage chamber is filled with laminate, the layers of which are conveyed into the extrusion head by the associated extruders which perform a material plasticising operation. In that case the annular piston which is used for emptying the storage chamber is thus moved back into its second limit position which corresponds to the end of the storage chamber-filling operation. During that phase of operation, no preform is ejected from the extrusion head so that the material in laminate form which is to be found in the duct system between the storage chamber and the outlet opening is also not moved. After the operation of filling the storage chamber with material has been concluded, there then follows the other phase of operation in which the previously formed laminate, for producing a preform, is ejected from the storage chamber under the effect of the annular piston and displaced towards the outlet opening. When that happens, the laminate flows through a communicating duct towards the outlet opening. During that second phase of operation, the ejection operation which forms the preform is predominantly effected by virtue of displacement of the material out of the storage chamber by means of the annular piston. However also added thereto is a minor portion of material which, when the extruder units are operating continuously, is conveyed thereby into the extrusion head during the ejection stroke movement of the piston, with the consequence that as a result a certain proportion of material is additionally urged towards the outlet opening, in addition to the material which is displaced by the annular piston. It will be appreciated that it is desirable for the extruders which are operable to plasticise the material for forming the respective layers of the laminate structure operate in an uninterrupted fashion in order that the operating conditions at the laminate-formation region within the apparatus can be stabilised. That is necessary in order for the individual layers of the laminate structure to be of the respectively desired thicknesses, thereby making it possible to maintain the proper distribution of thicknesses of the respective layers within the laminate structure.
The batch-wise and thus discontinuous production of preforms results in different operating conditions, corresponding to the different phases of operation, in particular in regard to the pressure conditions which obtain in the extrusion head. The different and accordingly varying pressure conditions mean that, at the beginning and possibly also at the end of each emptying stroke movement, and also at the beginning of each filling stroke movement, different pressure conditions obtain in the laminate-formation region of the apparatus, in regard to the individual flows of material which go to make up the laminate structure. The varying pressure conditions can result in irregularities in the formation of the laminate structure. That is to be attributed for example to the fact that the volumes of the individual layers of the laminate structure and therewith the volumes of the individual duct systems through which the flows of material, while still separate, flow to the laminate-formation region in the apparatus, are normally different. As thermoplastic materials, in the plastic condition, do not behave like a liquid but within certain limits have a certain degree of resilient compressibility which, when the pressure is relieved, results in a return movement, that is to say an increase in the volume of the plastic material, it will be noted that at the beginning and possibly also at the end of each emptying stroke movement and possibly also at the beginning of each filling stroke movement, the individual flows of material which make up the laminate structure experience deformation effects in the region of the apparatus in which the laminate structure is formed, due to differences in compression and decompression effects in respect of the flows of material. Such deformation of the flows of material will result in the laminate structure being of an irregular nature, in particular in regard to the spatial and quantitative distribution of the individual layers in the laminate structure. Those irregularities remain limited to the first phase of the emptying stroke movement as, after the beginning of the emptying stroke movement, a condition of equilibrium occurs in the system. A similar aspect may also apply in regard to the terminal phase of the emptying stroke movement and in regard to the initial phase of the filling stroke movement. Those two phases possibly blend into each other in such a way that the pressure-relief effect begins just shortly prior to the conclusion of the emptying stroke movement, so that a condition of equilibrium in respect of the pressures involved will be restored again only after the beginning of the filling stroke movement. At any event the pressure-relief effect can result in decompression of the material which will differ from one flow of material to another, by virtue of the differences in volume and properties of the materials forming the individual flows, whereby irregularities in the distribution of the individual layers in the laminate structure may occur in the region of the laminate-formation region. Those irregularities do not occur at least in that manner when the preforms are extruded continuously as that procedure does not involve any variations in the pressure conditions, as are caused by batch-wise formation of the preforms.
The above-discussed irregularities can result in serious reductions in the level of quality of the hollow bodies to be produced from the preforms, by virtue of the fact that the walls thereof are of an irregular nature. In that connection the way in which the flows of material are guided in the extrusion head prior to the step of forming the laminate therefrom is immaterial as the variations in pressure conditions, which give rise to such irregularities, do occur at any event due to the change between filling and emptying, which is typical in respect of batch-wise production of the preforms.